The only one officially designed to work with the Touchstone was HP's official case. I've heard many people say that Acase touchpad cases will work on a Touchstone, however. I've got the case, but I'm waiting for the Touchstone to come in the mail; I'll let you know when it does.

The Incipio Kickstand case for the TouchPad works with the Touchstone with any version of webOS. The Targus Truss case works with the Touchstone with 3.0.2 and 3.0.5 (update just came out)--it does not work with 3.0.4. The Targus case provides great protection--I only got the Incipio to use while I was waiting for the 3.0.5 update. Now that I've installed 3.0.5, I'll only use the Targus case.

Will warn you, however, that it only seems to work in portrait. Without the case, it works just fine in landscape. Also, it's not quite as nice aesthetically if it's in the case, since the outer flap has to be folded back somewhere.So you probably won't be able to use it to turn your touchpad into an impromptu picture frame unless you're willing to pull it out of the case. But if you just need to charge it, it will charge while encased.

There are two things that would keep a case from working with a Touchstone: distance and materials. Considering that the Touchstone will work through the folded-over back of the HP Folio case, distance isn't a huge concern unless the case is ridiculously thick. And so long as the case isn't made of something excessively dense, it should still charge fine. The biggest thing to avoid is, of course, metal.

"'Form follows function' — that has been misunderstood. Form and function should be one, joined in a spiritual union."
Frank Lloyd Wright

I'm loving my Poetic leather case, and it indeed works with the Touchstone. Much sturdier than the official HP case, and it helps the TP stand up better (in landscape mode). Highly recommended (and, no, I do not work for Poetic). http://amzn.com/B005J064XS